Onlookers were baffled by the gross, gooey mess. But a researcher at the California Academy of Sciences says that it's a commonplace, predictable event. As it happens, jellyfish tend to gather in large "blooms" offshore. They don't normally make it to land, but with just the right currents and winds, they'll get stranded on the beach.

Jellyfish Invade Ocean Beach

Ocean Beach is known for its unusual currents, though most humans know it more for the riptide than for jellyfish-killing swells. For once, there's no evidence that this mass die-off is the fault of human intervention.

But that doesn't mean that humans aren't to blame. A bloom this size is fairly rare, and could signal some kind of environmental change that led to a population boom. It could be a decrease in jellyfish predators, or an increase in jellyfish-friendly pollution. It's hard to say for sure, although a recent report did identify suburban chemicals as the largest source of pollution in the bay.

Images: Giant Jellyfish Spotted in San Diego Waters

The blobs are mostly gone now, either washed away or picked apart by scavengers or decomposed into the sand. It doesn't take much to dispense with an invertebrate.

And it's not just jellyfish putting in an appearance on the sand. The King Phillip, a 150-year-old shipwreck, has briefly emerged on the beach as well. Built in 1856, the ship was wrecked on the beach in 1878. Since then, it's sunk beneath the sand, only to re-emerge every few years as the sand shifts.